Eren and Zeke after their deaths were reincarnated into a world with demons instead of titans like titans they feasted on humanity, Zeke having a recollection of his past life unlike his younger brother sought them to become the strongest, when Eren is badly injured by a swordsman he does the only thing that would save his little brother's life and his, he became a demon. KNY x SNK

Tanjiro never thought that he would become a demon of course not he was a kind boy and wanted the best for his family but as he turned 13 he started to have urges, and these urges weren't normal, he was falling deathly in love with someone, and that someone was his father. As a demon Tanjiro relishes these urges as they run rampant he was now a demon urges he had are still there.